This invention relates to a system for feedback control of the air/fuel ratio in an internal combustion engine, usually an automotive engine, which is to be normally operated with a lean mixture. The control system includes means to vary the target value of the air/fuel ratio at least under predetermined transient operating conditions of the engine.
Recent automotive engines have to satisfy severe requirements as to high power performance, low exhaust emission and good fuel economy all together. One approach to the solution of problems relating to such conflicting requirements is operating the engine with a very lean air-fuel mixture under precise control of the fuel feed system.
For example, a lean combustion automotive engine system is described in "NAINEN KIKAN" (a Japanese journal), Vol 23, No. 12 (1984), 33-40. This system includes an air/fuel ratio feedback control system, which uses an oxygen-sensitive solid electrolyte device as an exhaust sensor to detect the actual air/fuel ratio in the engine, and a three-way catalyst which catalyzes not only oxidation of CO and HC but also reduction of NO.sub.x. The output of the exhaust sensor used in this system becomes nearly proportional to the actual air/fuel ratio over a wide range which extends from a slightly sub-stoichiometric ratio to an extremely super-stoichiometric ratio, so that feedback control of the air/fuel ratio can be performed with a widely variable target value. As a typical example, the target value of air/fuel ratio in the feedback control system is 21.5 during steadystate operation of the engine and changes to 22.5 under gently accelerating conditions, to 15.5 under idling conditions and to a sub-stoichiometric value in the range of about 12-13 under full-load operating conditions.
The use of a very lean mixture is very effective in reducing the emission of NO.sub.x to a level that meets the current regulations, though the three-way catalyst becomes less effective in reducing NO.sub.x when the engine is operated with either a very lean mixture or a very rich mixture. However, under steeply transient operating conditions of the engine it is impossible to realize the required power performance of the engine while maintaining a super-stoichiometric air/fuel ratio sufficient for reducing the emission of NO.sub.x. To continue the lean combustion even under steeply transient conditions without dissatisfaction in any aspect, it is necessary to further improve the precision and quickness of the feedback control of air/fuel ratio from the state of the art. Therefore, it is customary to shift the air/fuel ratio under steeply transient operating conditions of the engine from a super-stoichiometric value to a sub-stoichiometric value to thereby maintain the required power performance and driveability even though this measure causes the emission of NOx to increase beyond tolerance.